A wide variety of organisms (including certain species of snakes; spiders, scorpions and other arachnids; insects; fish; jellyfish; and lizards) are capable of producing venoms, and of transmitting them (“envenomation”) by either biting or stinging.
Rattlesnake venom is a complex mixture of toxic components (individually referred to as toxins), composed mainly of both enzyme and non-enzyme proteins. Toxins are divided into more than twenty different protein families that are defined by their molecular structure. While families of toxins share a general structure, there may be a variety of toxic activities represented by a single toxin family. Typically, an individual rattlesnake's venom will contain toxins representing a dozen toxin families and can contain upwards of 50 unique toxins. While lethality is a major concern in envenomation, other serious adverse events which are likely, include hemorrhage, tissue necrosis and systemic blood clotting, among others.
A vast majority of snakebite envenomation cases in the state of California result from encounters with individuals from the two subspecies of Western Rattlesnake, the Northern Pacific Rattlesnake (Crotalus oreganus oreganus) and the Southern Pacific Rattlesnake (Crotalus oreganus helleri). The venom of this species can cause a wide range of clinical issues, including tissue damage, blood clotting and effects on the nervous system.
One method of acutely treating domestic animals or humans that have been envenomated by venomous snakes or other venomous organisms is to administer an intravenous dose of monoclonal or polyclonal antibodies harvested from a mammalian species (e.g. sheep, horse, or goat) that has been immunized against a venom. Antibody-based therapeutics are known as antivenoms or antivenins.
Snake antivenom is “monovalent” if it is produced from the venom of a single species of snake and “polyvalent” if produced from the venom of multiple species of snakes.
While the use of antivenom is an acute treatment option used following an envenomation event, vaccination against venom is a prophylactic—for protecting an animal or human before they suffer envenomation. All toxins can be denatured by physical or chemical means to produce attenuated materials known as toxoids, which immunologically resemble the source toxin and can produce a protective immune response. While toxoids closely resemble the original toxins in their structure, the neutralization process(es) disrupt enough molecular structure to render the toxoids nontoxic.
The vaccination of a mammal against the effects of envenomation can be accomplished using either bioactive venom toxins or venom toxoids. Toxoids are preferred to minimize the risk of injury or death to the mammal from toxin. Toxoids are preferably administered at a dose and frequency to generate a strong and lasting immune response. Thus in designing a toxoid vaccine, the goal is to present the patient's immune system with enough toxoids to stimulate the immune system to defend against each of the potentially dangerous components (or its immunological equivalent) that might be transmitted in envenomation.
Snake venom is known to vary within and between species. There are more than 30 distinct species of venomous snakes in the contiguous U.S., each with a distinct venom composition compared with other species and some with distinct compositions between subspecies. Existing research also shows that snake venom varies within and between geographic populations of venomous snake. This immense amount of variation is thought to arise from locality specific evolutionary pressures acting on snake venom composition and affecting the ecological fitness of a population over time.
The Southern Pacific Rattlesnake (Crotalus oreganus helleri) ranges across some of the most heavily populated areas of Southern California. A number of geographically separate populations of the Southern Pacific Rattlesnake exhibit significant variations in venom composition, each with unique biochemical properties. The Northern Pacific Rattlesnake (Crotalus oreganus oreganus) ranges further north, and shows local variations in venom composition.
There is only one commercially-available snake venom vaccine (Canine and Equine Rattlesnake Toxoid Vaccine, from Red Rock Biologics, Woodland, Calif.) approved by the USDA-CVB for use in domesticated animals (dogs and horses). The vaccine is monovalent and is produced from a single species—the Western Diamondback Rattlesnake (Crotalus atrox). This product is referred to as “CAT Vaccine” (Crotalus atrox toxoid vaccine).
The CAT vaccine is a sub-lethal dose of a mixture of toxoids that causes the vaccinated mammal to mount an immune response and produce its own anti-venom antibodies against future snakebite. Antibodies produced by an animal immunized with the CAT vaccine have limited or no ability to protect against the venom of species of rattlesnake other than the Western Diamondback Rattlesnake (Crotalus atrox). See Cates et al. (2015), Comparison of the protective effect of a commercially available Western Diamondback Rattlesnake toxoid vaccine for dogs against envenomation of mice with Western Diamondback Rattlesnake (Crotalus atrox), Northern Pacific Rattlesnake (Crotalus oreganus oreganus), and Southern Pacific Rattlesnake (Crotalus oreganus helleri) venom, Am. J. Vet. Res. 76(3):272-9.
The failure of this CAT Vaccine to protect mice, an experimental model mammal, when experimentally envenomated with the venom from a Southern Pacific Rattlesnake (Crotalus oreganus helleri) presumably results from differences between the venom used to formulate the CAT vaccine and the venom of the Southern Pacific Rattlesnake.
The derived heterodimeric lectin toxins (α- and β-chains) characteristic of viper venoms, which exhibit a diversity of biological activities including anticoagulation and agonism/antagonism of platelet activation or procoagulation, are both absent from the San Jacinto Mountain population, but are present in all other populations of the Southern Pacific Rattlesnake. The extreme variation of venom composition between the different populations of Southern Pacific Rattlesnake and the absence of the neurotoxin phospholipase A2 complex in the CAT vaccine renders the CAT vaccine ineffective against not only all of the Southern Pacific Rattlesnake populations, including the San Jacinto Mountain population, but also against envenomation from other species of rattlesnake including the Mojave Rattlesnake (Crotalus scutulatus).
Thus, there is currently a great need for polyvalent and broadly protective venom toxoid vaccines in the regions where dangerous snakes reside. A broadly protective vaccine is expected to significantly reduce mortality of dogs and horses from snakebite envenomation, significantly reduce the suffering of canine and equine patients and their owners, and significantly reduce the financial burden to dog and horse owners by reducing the duration of treatment and hospitalization of envenomated animals.